Hydrothermal carbonization (HTC) of carbohydrates and biomass is a straightforward method for preparing hydrochars at low temperatures of 180-250 °C. Hydrochars are more carbonized than their precursors. Increasing the carbonization degree of hydrochars at hydrothermal temperatures is a scientific quest that is addressed in this thesis. Hydrochars are known to have a spherical or irregular morphology. Here we address thin film hydrochars for the first time.  Hydrochars themselves are carbon precursors for preparing activated carbons. Activated carbons are porous materials that can be used for gas adsorption applications. In this thesis, enhanced adsorption of VOCs at low pressures is addressed by using iron phosphate impregnated activated carbons. Finaly, any chemical process or product including those in this thesis such as HTC, activation, hydrochar and activated carbons may contribute to the issue of environmental degradation positively or negatively. Such environmental impacts are addressed by life cycle assessment of processes of HTC and activation and their related products in the last paper of this thesis. Briefly mentioned, in my first study (Paper I), I focused on the HTC of glucose in the presence of iron (II) sulfate. By changing the concentration of iron (II) sulfate, with a catalytic amount, blue hydrochars were formed at the bottom of the autoclave. The blueness was related to thin film interference. The thin film hydrochars were more carbonized than spherical hydrochars and the yield of HTC has increased in the presence of iron (II) sulfate. The second study (Paper II) is focused on the activation of hydrochars with H₃PO₄ and H₃PO₄+FeCl₃. We showed that ultramicroporosity and impregnated iron phosphate species enhance the adsorption of VOCs at low pressure. The ACs were impregnated with Fe (PO₃)₂ and it was shown that Fe (PO₃)₂ acts as an activation agent which opens up for future studies. In the third study (Paper III), H₃PO₄-activated carbons were prepared and modified with FeS and FeSe and it was shown that the ACs were also impregnated with Fe₂P, in the case of AC-FeS/Fe₂P. FeSe and FeS were not detected by XRD. Only large crystals of Fe₂P were detected in the sample AC-FeS/Fe₂P. In the last study (Paper IV), prickly pear seed biomass from the agro sector in Tunisia was hydrothermally carbonized. The hydrochars were then activated into ACs by CO₂ activation. The life cycle assessment of the HTC and activation process was investigated.  

Volatile organic compounds (VOCs) are often hazardous and need commonly to be removed from gas mixtures. Capture on activated carbon (AC) is one approach to achieving this. We hypothesized that the smallest pores on ACs and the inclusion of inorganic phosphates could enhance the low gas pressure uptake of two typical VOCs (acetone and isopropanol). To test this hypothesis, ACs were prepared by chemical activation of hydrochars with H3PO4 or a mixture of FeCl3 and H3PO4. The hydrochars had been prepared by hydrothermal carbonization of glucose. The ACs were characterized by XRD, IR, TGA, and the adsorption of N2, CO2, H2O, acetone, and isopropanol. The results showed that the ACs had comparably high adsorption of acetone and isopropanol at low vapor pressures. The low-pressure uptake (at 0.03 kPa) of isopropanol and acetone had values of up to 3.4 mmol/g and 2.2 mmol/g, respectively. This suggests that ACs containing iron phosphate could be of relevance for adsorption driven removal of VOC. It was also observed that the external surface area of the ACs containing iron phosphates increased upon secondary heat treatment in N2.

The agro-sector generates organic waste of various kinds, which potentially could be used to prepare functional materials, lessen environmental problems, and enhance circularity. In this context, the hypothesis that was put forward in this work is that prickly pear seed waste from the Tunisian agro-food industry could be used to prepare activated carbons. The prickly pear seed waste was first hydrothermally carbonized and the resulting hydrochar was activated in CO₂ at 800 °C. The yield of the hydrothermal carbonization process is of importance, and it was the highest at intermediate dwell times and temperatures, which was ascribed to the re-precipitation of hydrochar particles on the heat-treated biomass. The hydrochars and activated carbons were characterized with scanning electron microscopy, thermogravimetry, Raman spectroscopy, and N₂ and CO₂ adsorption/desorption analyses. The activated carbons had micro- (<2 nm) and mesopores (2–50 nm), and specific surface areas and total pore volumes of about 400 m^2 −1 and 0.21 cm³ g⁻¹. The study showed that the prickly pear seed waste could be effectively transformed into both hydrochars and activated carbons and that is advisable to optimize the hydrothermal process for the mass yield. A life cycle analysis was performed to assess the environmental impact of the production of typical activated carbons using the approach of this study. Further studies could be focused on enhancing the properties of the activated carbons by further optimization of the activation process.

We hypothesized that the morphology of the hydrochar from hydrothermal carbonization of glucose would be affected by Fe²⁺; and indeed, with such ions, large pieces of hydrochar formed that comprised aggregated spherical particles and blue and thin films. Thin carbonized films formed at the bottom of the autoclave liners or on Teflon^TM tape used as a template. Free-standing films could be prepared by stretching the Teflon^TM tape after the synthesis. The carbonized films that formed at the bottom of the autoclave adhered to spherical hydrochar particles. The blueness was ascribed to thin-film interference under white-light irradiation and related to the film thickness, which was about 200 nm. Analysis of transmission electron microscopy (TEM) images showed that the films consisted of a layered amorphous carbon. The amorphous and thin films were more carbonized than the amorphous carbon of the TEM grid, as derived via electron energy loss spectroscopy (EELS). Additional analysis of one of the thin films by X-ray photoelectron spectral analysis showed a higher carbon fraction than for bulk hydrochar, supporting the EELS analysis. We believe that the synthesis of thin films of hydrochar can open up new colloidal processing pathways, which could be useful in the preparation of carbon-based materials and alike.

Global concerns regarding climate change and the energy crisis have stimulated, among other things, research on renewable and sustainable materials. In relation to that, hydrothermal carbonization of wet biomass has been shown to be a low-cost method for the production of hydrochars. Such hydrochars can be refined into materials that can be used in water purification, for CO2 capture, and in the energy sector. Here, we review the use of metal ions and particles to catalyze the formation of hydrochars and related hybrid materials. First, the effects of using silver, cobalt, tellurium, copper ions, and particles on the hydrothermal carbonization of simple sugars and biomass are discussed. Second, we discuss the structural effects of iron ions and particles on the hydrochars in conjunction with their catalytic effects on the carbonization. Among the catalysts, iron ions or oxides have low cost and allow magnetic features to be introduced in carbon-containing hybrid materials, which seems to be promising for commercial applications.